Metal hydroxides are a raw material for the production of metal oxides, which represent an important basic substance of inorganic chemistry. In their occurrences in nature, metal hydroxides chiefly are present in a mixed form, so that the raw materials must be cleaned up.
In the case of the production of aluminum hydroxide, this is accomplished by the so-called Bayer process, in which the mined minerals, mostly bauxite, are comminuted and impregnated with sodium hydroxide solution. Insoluble residues, such as red mud which chiefly contains iron oxide, thus can be separated from the dissolved aluminum hydrate by filtration, thickening or other means. By crystallization and further filtration, pure aluminum hydroxide (Al(OH)3) is obtained from this solution.
A process for producing alumina (Al2O3) from aluminum hydroxide is described for example in EP 0 861 208 B1 or DE 10 2007 014 435 A1. The filter-moist aluminum hydroxide initially is dried in a first suspension heat exchanger and preheated to a temperature of about 160° C. Upon separation in a cyclone separator, the solids are supplied to a second suspension preheater, in which they are further dried with the waste gas from the recirculation cyclone of a circulating fluidized bed, and are then charged to a fluidized-bed reactor of the circulating fluidized bed. In the fluidized-bed reactor, the aluminum hydroxide is calcined to alumina at temperatures of about 1000° C. A partial stream of the preheated aluminum hydroxide is branched off after the first suspension preheater (EP 0 861 208 B1) or after the second suspension preheater (DE 10 2007 014 435 A1) and mixed with hot alumina withdrawn from the recirculation cylcone of the circulating fluidized bed. The hot product mixture subsequently is cooled in a multi-stage suspension cooler in direct contact with air and then supplied to a fluidized-bed cooler for final cooling. The fluidization of the fluidized bed in the fluidized-bed reactor is effected by means of fluidizing gas (primary air), which in a first chamber of the fluidized-bed cooler has been preheated to a temperature of 188° C. In the suspension heat exchangers for cooling the product, secondary air additionally is heated to 525° C. in a direct heat exchange with alumina, before it is supplied to the fluidized-bed reactor.
EP 0 245 751 B1 describes a process for performing endothermal processes on fine-grained solids, with which the product heat within the entire process should be utilized in a more efficient way. During the calcination of aluminum hydroxide, a partial stream of the starting material is supplied to an indirectly heated preheater and subsequently introduced into an electrostatic precipitator together with the directly supplied feedstock. The solids then are supplied from the electrostatic precipitator via two series-connected preheating systems to a circulating fluidized bed, in which the solids are fluidized with fluidizing gas (primary air) and calcined at temperatures of about 1000° C. The stream of solids withdrawn from the circulating fluidized bed is cooled in an indirect fluidized-bed cooler forming a first cooling stage and then supplied to second and third cooling stages, again in the form of fluidized-bed coolers, in order to further cool the solid product. The primary air heated in the first fluidized-bed cooler is introduced into the fluidized-bed reactor as fluidizing air with a temperature of 520° C., whereas the fluidizing air of the fluidized-bed coolers is fed into the fluidized-bed reactor as secondary air with a temperature of 670° C. The heat transfer medium of the second fluidized-bed cooler is supplied to the indirect preheater as the heating medium for the feedstock with a temperature of 200° C. and then recirculated to the inlet of the second fluidized-bed cooler upon cooling to 160° C.
The calcination of aluminum hydroxide requires very much energy. Conventional processes require an expenditure of energy of about 3000 kJ/kg of alumina produced.